1. Field of the Invention
The present invention is directed to a magnetic resonance imaging tomography devices, of the type having a transmission antenna for exciting nuclear spins in an examination subject and a reception antenna in the form of a local coil.
2. Description of the Prior Art
Magnetic resonance imaging devices are known for use in producing tomograms of an examination subject, such as a human body. Such magnetic resonance imaging devices include a basic or fundamental field magnet, which aligns the nuclear spins in the examination subject, and also contain gradient coils which generate respectively spatially different magnetic fields, as well as high-frequency antennas for exciting the nuclear spins and for receiving the signals emitted by the excited nuclear spins. If a high-frequency antenna is used which contains an excitation and measuring coil, the inductance of the coil together with a variable capacitance are combined as an LC resonant circuit. The capacitance is then varied to tune the resonant frequency of the circuit to the desired frequency.
A known embodiment of such a magnetic resonance imaging apparatus has a superconducting basic field magnet, in the form of a solenoid, which generates a basic field extending in the direction of the cylinder axis. The excitation coil of the antenna in this known system may, for example, be formed by conductors which extend parallel to the cylinder axis and are arranged in a so-called envelope pipe, which represents a high-frequency shield consisting of material having good electrical conductivity which is frequency-transmissive for the low-frequency gradient fields but is non-transmissive for the high-frequency antenna fields. The ends of these conductors are respectively connected to the envelope pipe via at least one resonant capacitor. A high-frequency equiphase field is formed between the conductor and the envelope pipe. The envelope pipe projects beyond the end faces of the conductors, so that a circular hollow waveguide antenna having aperiodic wave propagation is achieved, whose coupling elements are the conductors of the excitation coil of the antenna. Such a known arrangement is described in U.S. Pat. No. 4,680,550.
It is also known to make use of so-called surface coils or local coils for generating images of small body sections. A good signal-to-noise ratio is obtained with such local coils, because noise signals can only be received from a comparatively small body region. In the simplest case, such surface coils consist of a circular wire loop adapted for handling high-frequency signals. Since it is necessary that the excitation field which excites the nuclear spins be as homogenous as possible, and since a local coil could not achieve the desired homogeneity, a whole-body antenna is used as the excitation coil, for generating a high-frequency field having good field homogeneity. The local coil is then used solely for reception purposes. In this type of antenna system, therefore, two different, resonant high-frequency antennas participate in the imaging.
In order to avoid coupling of the high-frequency alternating fields, generated by the transmission antenna, with the reception antenna, the reception antenna can be detuned during transmission and the transmission antenna can be detuned during reception. Such detuning is achieved by DC-controlled high-frequency switches, preferably by a circuit having PIN diodes as disclosed in U.S. Pat. No. 4,801,885. Coupling via the high-frequency magnetic fields is thereby reduced. In addition, however, coupling of the electric field of the transmission antenna with the feeder for the local coil may also occur.